Motor vehicles having an electric motor such as electric cars or hybrid vehicles are known. In particular batteries using lithium-ion technology are suitable as energy storage devices for electric vehicles. These batteries are characterized amongst other characteristics by virtue of a high energy density and also an extremely low self-discharge. Vehicles can be driven by means of an electric machine that is embodied as a multiphase, preferably three-phase, electric motor and is supplied by the traction battery, in particular the lithium-ion traction battery.
Currently, electric vehicles generally have shorter ranges of travel than vehicles having an internal combustion engine as a result of a limited battery capacity and a limited charge rate and said electric vehicles are often only ready for further use after a comparatively longer charging period of by way of example one to two hours. For this reason it is often necessary to fall back on so-called “range extenders” (REX). The term “range extender” (also described as a “device that extends the range of the vehicle”) is understood to mean in the case of an electric vehicle an additional unit with which the range of the vehicle can be increased. In this manner, it is possible to use an internal combustion engine, which is installed in an electric vehicle, for the purpose of driving a generator by means of which the traction battery is charged. Alternatively or in addition, the energy from the internal combustion engine can also be used to drive the vehicle. It is possible for the purpose of charging the battery to select a uniform operation for the internal combustion engine and the generator and consequently an optimal operating point is maintained, whereby the level of efficiency of the range extender is improved.
DE 10 2009 001 705 A1 discloses an electric vehicle having a battery charging device in which an internal combustion engine that is arranged as a range extender is started by means of energy that is provided by means of the battery charging device. The battery charging device is preferably embodied in a bidirectional manner.
DE 10 2007 004 172 A1 discloses a drive system for a vehicle that comprises an electric motor. In addition, an internal combustion engine and a generator that is connected in series to said internal combustion engine is provided and the traction battery of the electric motor is charged by means of said generator.
DE 10 2010 031 159 A1 discloses a range extender in the form of an internal combustion engine and an asynchronous machine. In the generator operation, the traction battery is charged by way of an interconnected rectifier by means of the asynchronous machine that is driven by means of an internal combustion engine. In the motor driven operation, the asynchronous machine is supplied by a traction battery by way of an inverter and can be used to start and accelerate the internal combustion engine.
In addition, traction batteries having multiple, in particular three, battery strings have been disclosed in earlier patent applications of the applicant. These battery strings can be switched in such a manner that they are phase-offset with a specific frequency, wherein each of the battery strings supplies one of the connecting terminals of a multiphase electric motor. Batteries of this type having multiple battery strings that are operated in a manner phase-offset with respect to one another are also described as battery direct inverters.
If a drive system having a multi-string traction battery of this type is operated with a range extender, the difficulty or rather the problem however that is to be overcome is that not only one but rather by way of example three battery strings are provided that all need to be charged.